Method for performing frequency synchronization of a base station and a network part

ABSTRACT

The invention relates to a method for performing frequency synchronization of a base station, and to a network part. In the method, the following operations are performed: maintaining a reference clock in a network element of the cellular radio network; generating a time stamp signal in the reference clock; transferring the time stamp signal from the network element to the base station over an asynchronous data transmission connection; calculating on the basis of the transferred time stamp signal how much the time by the local clock at the base station deviates from the time by the reference clock; generating a speed correction factor for the local clock on the basis of at least one calculated deviation; correcting the running of the local clock with the speed correction factor; and generating the frequencies needed at the base station by using the local clock corrected with the speed correction factor.

[0001] This application is a Continuation of International ApplicationPCT/F100/00750 filed on the Sep. 6, 2000 which designated the U.S. andwas published under PCT Article 21(2) in English.

FIELD

[0002] The invention relates to a method for performing frequencysynchronization of a base station in a cellular radio network, and to anetwork part in a cellular radio network.

BACKGROUND

[0003] A base station of a cellular radio network requires an accurateclock signal to guarantee high frequency stability and accurate timingon the air interface. GSM specifications require a relative accuracy of5×10⁻⁸ on the air interface which might be relaxed to 10⁻⁷ for basestations used in a pico cellular environment. This high accuracy isachieved by conveying a clock signal as a pulse train along the nationaltelephone backbone, along the GSM infrastructure, e.g. Mobile SwitchingCentre (MSC) or Base Station Controller (BSC) up to the base stations.

[0004] The national reference clock has a relative frequency stabilityof 10⁻¹¹ over 24 hours. But the long transmission chain to the basestation introduces jitter and wander in the clock signal. The basestation relies on an accuracy of 1.5×10⁻⁸ at its 2 MBit/s PCM (PulseCode Modulation) Abis interface. The transcoder inside the base stationhas a 16 MHz clock (divided down to 2 MHz) which is phase locked to thePCM clock pulses, jitter and wander above 2 Hz is filtered out, and thesignal is averaged over approximately 15 minutes. The 2 MHz clock signalwhich has been “cleaned” in this way has an improved accuracy and servesas a reference clock for a 26 MHz clock in the Base Station ControllerFunction (BCF). All frequencies and timing on the radio interface areultimately derived from this 26 MHz clock.

[0005] The described known method of providing the base station with anaccurate clock relies on a continuously existing transmission chain fromthe fixed network to the base station. This becomes a problem if part ofthis transmission chain runs across a non-clocked network, which is thecase for the new indoor cellular radio networks. In these networks thereusually is no BSC but the functionality of the BSC is distributed overan IP (Internet Protocol) network, or intranet. IP networks are notclocked since they operate asynchronously, and transmission times arehighly variable and unpredictable.

[0006] One solution to the problem is to equip a network element with ahighly accurate clock and the clock signal is distributed to a basestation with a synchronous line, e.g. an ISDN (Integrated ServicesDigital Network) or HDSL (High Bit Rate Digital Subscriber Line)transmission line.

[0007] The goal for the indoor cellular radio networks, however, is totake advantage of the existing network cabling in office environments byconnecting base stations directly to an asynchronous network.

[0008] Providing additional cables for carrying a clock signal worksagainst the primary reason for using the intranet: making better use ofan existing network. With additional cables there is no need to connectbase stations to the LAN (Local Area Network) at all. Base stations canthen be directly connected to the network via HDSL transmission which issynchronous and which only requires a simple twisted pair cable.

[0009] There exists a wide variety of clocks which can be used at basestations. Very expensive clocks require a constant temperatureenvironment (oven maintained) and provide a high accuracy approachingeven that of the national reference clock. To increase the costefficiency of the system, expensive, oven maintained clocks should beavoided as far as possible, especially at base stations.

BRIEF DESCRIPTION

[0010] An object of the invention is to provide an equipment that allowsthe above problems to be solved. This is achieved with an equipmentdescribed below, which is a network part in a cellular radio networkcomprising: a base station, the base station comprising a local clock; anetwork element connected to the base station via an asynchronous datatransmission connection, the network element comprising a referenceclock; the reference clock comprising means for generating a time stampsignal and means for sending the time stamp signal over the asynchronousdata transmission connection from the network element to the basestation; the base station comprising means for receiving the time stampsignal sent over the asynchronous data transmission connection and meansfor calculating on the basis of the received time stamp signal how muchthe time by the local clock at the base station deviates from the timeby the reference clock. The base station further comprises means forgenerating a speed correction factor for the local clock on the basis ofat least one calculated deviation; means for correcting the running ofthe local clock with the speed correction factor; and a frequencysynthesizer for generating the frequencies needed at the base station byusing the local clock corrected with the speed correction factor.

[0011] The invention further relates to a method for performingfrequency synchronization of a base station in a cellular radio network,the method comprising the steps of maintaining a reference clock in anetwork element of the cellular radio network; generating a time stampsignal in the reference clock; transferring the time stamp signal fromthe network element to the base station over an asynchronous datatransmission connection; calculating on the basis of the transferredtime stamp signal how much the time by the local clock at the basestation deviates from the time by the reference clock. The method alsocomprises the steps of generating a speed correction factor for thelocal clock on the basis of at least one calculated deviation;correcting the running of the local clock with the speed correctionfactor; generating the frequencies needed at the base station by usingthe local clock corrected with the speed correction factor.

[0012] The basic idea of the invention is to control the running of thebase station clock on the basis of the time data provided by the timestamps.

[0013] The method and equipment of the invention provide severaladvantages. An expensive clock is not needed at the base station, whichreduces the costs of manufacture of the base station. In addition, asynchronous data transmission connection is not needed for transmittingtime stamp signals, but an asynchronous data transmission connection issufficient. In the functioning of the method it is not the duration ofthe transmission delay but the stability of the delay variation that isessential.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the following the invention will be described in greaterdetail in connection with preferred embodiments and with reference tothe accompanying drawings in which

[0015]FIG. 1 illustrates an example of a structure of a cellular radionetwork;

[0016]FIG. 2 illustrates a transceiver structure;

[0017]FIG. 3 illustrates an example of a cellular radio network of theinvention; and

[0018]FIG. 4 is a flow chart illustrating measures of a method forperforming frequency synchronization of a base station.

DESCRIPTION OF EMBODIMENTS

[0019] With reference to FIG. 1, a typical structure of a cellular radionetwork will be described. FIG. 1 only comprises the blocks that areessential for the description of the invention, although it is apparentto a person skilled in the art that a conventional cellular radionetwork also comprises other functions and structures which need not bediscussed in greater detail in this context. The example illustrates aGSM cellular radio network utilizing TDMA (Time Division MultipleAccess), without, however, restricting the invention thereto.

[0020] A cellular radio network typically comprises a fixed networkinfrastructure, i.e. a network part, and subscriber terminals 150, suchas fixedly mounted, vehicle mounted or portable terminals. A subscriberterminal 150 can be for instance a standard mobile phone which can beconnected by means of an additional card to a portable computer, forexample, that can be used in packet transmission for ordering andprocessing of packets.

[0021] The network part comprises base stations 100. A plural number ofbase stations 100 are, in turn, controlled in a centralized manner by abase station controller 102 communicating with them. A base station 100comprises transceivers 114. A base station 100 typically comprises 1 to16 transceivers 114. One transceiver 114 offers radio capacity to oneTDMA frame, i.e. typically to eight time slots.

[0022] The base station 100 comprises a control unit 118 which controlsthe operation of the transceivers 114 and a multiplexer 116. Themultiplexer 116 arranges the traffic and control channels used by aplural number of transceivers 114 to a single transmission connection160. The transceivers 114 of the base station 100 are connected to anantenna unit 112 which provides a bi-directional radio connection 170 toa subscriber terminal 150. The structure of the frames transmitted inthe bidirectional radio connection 170 is determined in detail, and theconnection is referred to as an air interface.

[0023]FIG. 2 illustrates in greater detail the structure of atransceiver 114. A receiver 200 comprises a filter blocking frequenciesoutside a desired frequency band. A signal is then converted to anintermediate frequency or directly to baseband, and in this form thesignal is sampled and quantized in an analog-to-digital converter 202.An equalizer 204 compensates for interference caused for instance bymulti-path propagation. From the equalized signal, a demodulator 206takes a bit stream, which is transmitted to a demultiplexer 208. Thedemultiplexer 208 separates the bit stream from different time slotsinto separate logical channels. A channel codec 216 decodes the bitstreams of the separate logical channels, i.e. decides whether a bitstream is signalling data, which is transmitted to a control unit 214,or whether the bit stream is speech, which is transmitted 240 to aspeech codec 122 of the base station controller 102. The channel codec216 also performs error correction. The control unit 214 performsinternal control functions by controlling different units. A burstformer 228 adds a training sequence and a tail to the data arriving fromthe speech codec 216. A multiplexer 226 assigns a time slot to eachburst. A modulator 224 modulates digital signals to a radio frequencycarrier. This is an analog operation, therefore a digital-to-analogconverter 222 is needed for performing it. A transmitter 220 comprises afilter restricting the bandwidth. In addition, the transmitter 220controls the output power of a transmission. A synthesizer 212 arrangesthe necessary frequencies for the different units. The synthesizer 212comprises a clock which in the invention is controlled from anothernetwork element, for example from the base station controller 102. Thesynthesizer 212 generates the necessary frequencies by using a voltagecontrolled oscillator, for example.

[0024] As shown in FIG. 2, the structure of the transceiver can befurther divided into radio frequency parts 230 and a digital signalprocessor including software 232. The radio frequency parts 230 comprisethe receiver 200, transmitter 220 and synthesizer 212. The digitalsignal processor including the software 232 comprises an equalizer 204,demodulator 206, demultiplexer 208, channel codec 216, control unit 214,burst former 228, multiplexer 226 and modulator 224. Theanalog-to-digital converter 202 is needed for converting an analog radiosignal to a digital signal and, correspondingly, the digital-to-analogconverter 222 is needed for converting a digital signal to an analogsignal.

[0025] The base station controller 102 comprises a switching field 120and a control unit 124. The switching field 120 is used for switchingspeech and data and for connecting signalling circuits. The base station100 and the base station controller 102 form a Base Station System whichadditionally comprises a transcoder 122. The transcoder 122 convertsdifferent digital speech coding modes used between a public switchedtelephone network and a radio network, to make them compatible with eachother, for instance from the 64 kbit/s fixed network form to anothercellular radio network form (such as 13 kbit/s), and vice versa. Thetranscoder 122 is usually located as close to a mobile switching centre132 as possible because this allows speech to be transmitted between thetranscoder 122 and the base station controller 102 in a cellular radionetwork form, which saves transmission capacity. The control unit 124carries out call control, mobility management, collection of statisticaldata and signalling.

[0026]FIG. 1 illustrates how a circuit-switched transmission connectionis established between the subscriber terminal 150 and a Public SwitchedTelephone Network terminal 136. In the Figures, a line illustrates howdata travels through the system over the air interface 170, from theantenna 112 to the transceiver 114 and from there multiplexed in themultiplexer 116 over the transmission connection 160 to a switchingfield 120, where a connection has been established to an output leadingto the transcoder 122, and from there further through a connectionestablished at the mobile services switching centre 132 to the terminal136 connected to the public switched telephone network 134. At the basestation 100 the control unit 118 controls the multiplexer 116 performingthe transmission, and at the base station controller 102 the controlunit 124 controls the switching field 120 to ensure correct switching.

[0027] The invention is particularly well suited for use in cellularradio networks located in offices. The base stations 100 can in thiscase be called office base stations. A major advantage offered bycellular radio networks placed in offices is that they allow thetelecommunications network within the building to be used free of chargefor providing the transmission connection 160 between the base stations100 and the base station controller 102. The telecommunications networkcan be for example an IP network (Internet Protocol) or an ATM network(Asynchronous Transfer Mode). When for example an IP network is used,each network element can have a separate IP address to which the datapackets are addressed. The telecommunications network can also be alarger company-internal network, an intranet, connecting the company'sgeographically separate offices together.

[0028] As already stated above, indoor cellular radio networks do notnecessarily have a network element called base station controller.Instead, the functionality provided by a base station controller can bedispersed into network elements connected with each other via anasynchronous data transmission connection, whereby the controller iscomposed of two computers, for example, that together provide the normalbase station controller functionality in the telecommunications networkand, in addition, the necessary management of the telecommunicationstraffic.

[0029] According to the invention, a network element of the cellularradio network, e.g. the base station controller 102 sends the timestamps, and the base stations directly receive them and each basestation generates its own reference frequency.

[0030] Another solution is that a LAN node element, e.g. a hub, bridge,router or switch, which is modified for use in a LAN dedicated to theindoor cellular radio network, sends the time stamps, and the basestations directly receive them and each base station generates its ownreference frequency. Especially the hub provides the advantage of lesscollisions occurring in the traffic, and therefore the time stampsending functions more reliably.

[0031] The base station, in turn, can be equipped with a cheaper clock.More expensive clocks might be required in the units sending the timestamps, but in any case fewer of them are needed.

[0032] There are different choices as to where the network elementsending time stamps in turn can receive an accurate reference from:

[0033] It can receive the clock signal generated by the nationalreference clock which is “visible” in the cellular radio network throughits A interface, i.e. the interface towards the MSC 132. Then, theincoming clock pulses are averaged in a similar way as at a base stationof the prior art. Thus, the clock in the time stamp sending unit canachieve an accuracy of at least 1×10⁻⁸.

[0034] The national reference clock might also be visible over someother telecom line, such as an E1/T1 connection to an Internet ServiceProvider.

[0035] The time stamp sending unit can have a built-in stand-aloneclock, especially if the loss of accuracy over the IP network is largerthan anticipated here, or there is no visibility to the nationalreference clock (e.g. a stand-alone indoor cellular radio networkwithout an A-interface). A clock with an accuracy of 1×10⁻⁹ might benecessary, for example an atomic clock, or a GPS (Global PositioningSystem) clock, in which case the GPS receiver's antenna might have to belocated outside the building.

[0036] It might be sufficient to have one stand-alone clock per anindoor cellular radio network system or building, if the time stampsending units can easily be connected to the stand-alone clock by cable,e.g. if these units are kept together in an equipment room.

[0037] Another possibility is a solution in which the clock signaltransmitted by an external base station over the air directly serves asa reference clock. Here, the time stamp sending unit can synchronise thefrequency of its clock over the air with this method.

[0038] The above solutions have the advantage that one network elementsending the time stamps can serve multiple base stations, since basestations do not have to be located close to the network element.

[0039]FIG. 3 illustrates an example of the structure of a network partin a cellular radio network of the invention. The rectangle drawn withthe dashed line on the right illustrates the structures of the basestation 102 that are of interest in the invention. The base station 102comprises a local clock 330.

[0040] The rectangle drawn with the dashed line on the left illustratesa network element 300 connected to the base station 102 via anasynchronous data transmission connection 312. The network element 300comprises a reference clock 302. The asynchronous data transmissionconnection 312 is in fact the same as the data transmission connection160 shown in FIG. 1.

[0041] The reference clock 302 comprises means for generating 306 a timestamp signal, and means for sending 310 the time stamp signal 308 overthe asynchronous data transmission connection 312 from the networkelement 300 to the base station 102.

[0042] The time stamp receiving unit, i.e. the base station 102,periodically requests 336 time stamps 312 which indicate the differencebetween two consecutive points in time when a time stamp was generated.

[0043] The request decision is thus decentralised. Another possibilityis that the time stamp sending unit, i.e. the network element 300, sendsthe time stamps 312 automatically without a specific time stamp request336.

[0044] The base station 102 comprises means for receiving 314 the timestamp signal 308 sent over the asynchronous data transmission connection312, and means for calculating 316 on the basis of the received timestamp signal 308 how much the time 332 by the local clock at the basestation 102 deviates from the time 304 by the reference clock 302.

[0045] The time stamp signal 308 is naturally also used for changing thetime 332 by the local clock at the base station 102 to correspond to thetime by the reference clock 302. This is necessary for the functioningof the method, otherwise it would not be necessary to synchronize theclocks, because in a cellular radio network transmissions of differentbase stations usually are mutually asynchronous.

[0046] The base station 102 further comprises means for generating 322 aspeed correction factor 324 for the local clock 330 on the basis of atleast one calculated deviation 318; means for correcting 326 the runningof the local clock 330 by using the speed correction factor 324; and afrequency synthesizer 212 for generating the frequencies needed at thebase station 102 by using the local clock 330 corrected with the speedcorrection factor 324.

[0047] The speed correction factor 324 thus informs how the running ofthe local clock 330 is to be corrected in the future in order to make itrun more precisely. In other words, the time of the local clock 330 isnot momentarily changed but the running rate of the clock iscontinuously controlled. Local clock 130 maintains the required accuracyfor at least 50-100 hours. In the local clock 130 a differential voltagecontrol mechanism is used to change the speed of the clock. A speedcorrection factor 324 is derived from the difference between the actualspeed of the local clock 330 and the speed of the reference clock 302according to the received time stamps 312.

[0048] In a preferred embodiment the speed correction factor 324indicates when the supply voltage of the local clock 330 changes, so theactual correction of the clock speed is performed by a differentialcontrol voltage. The application of this differential voltage isaccurate to about 80% since the relative change in voltage is very smalland the relationship between voltage and clock speed is not perfectlylinear. Iteration provides the means to correct the clock withprecision; 5-10 iterations should be sufficient. The total time for oneiteration procedure should be much shorter than the typical time scaleon which the characteristics of the differential voltage control circuitchange (which is about one year). The speed correction factor 324 thuscompensates for changes that gradually take place in the characteristicsof the local clock 330. Therefore the speed correction factor 324 ispreferably adjusted more than once a year.

[0049] A time stamp can be requested once every 24 hours during a lowtraffic period (e.g. at night). This is just an example which is used inthe calculations below. The value is set in the network element 300 byservice personnel according to the specific delay variabilitycharacteristics of the intranet.

[0050] The time stamps are delayed on the IP network, but for thismethod the variability of these delays is the relevant quantity. For atarget accuracy of 2×10⁻⁸ the delay variability has to be less than twomilliseconds.

[0051] In a preferred embodiment the base station 102 comprises means320 for calculating the variation in the transmission delay of a timestamp signal 324 sent over the asynchronous data transmission connection312, for comparing the deviation to a predetermined limit, and to deductthat the accuracy of the local clock does not meet the required level ifthe deviation exceeds the predetermined limit. The limit can be forexample the above mentioned two milliseconds. If the accuracy of theclock meets the required accuracy, no measures are needed, but if not,then an alarm 338, for example, can be raised at the management systemmonitoring the cellular radio system.

[0052] For the transmission of the time stamps an established protocolsuch as the network time protocol (NTP) might be used. The time stampemitting units act as NTP server, and the time stamp receiving units actas NTP clients. NTP measures the transmission delay and corrects thetime stamp in an iterative process which can last for a couple ofseconds. The protocol does not have to be IP-based as NTP, but couldalso directly build on top of the Ethernet link layer.

[0053] A higher rate of time stamps does not increase the accuracy ofthis method. But a rate of, for example, one time stamp per hour allowsthe time stamp receiving units to estimate the delay variability and tovalidate that the restrictions outlined above are met and to generate analarm otherwise.

[0054] The invention enables to reduce the number of expensive clocks inthe office system considerably, since a number of base stations canreceive a reference frequency from the same time stamp sending unit. Oneexample of such a scenario is that in one building there is one networkelement (which can even be a base station) which uses the externalnetwork as a reference clock. This reference network element sends timestamps to other base stations deeper inside the building. Within onebuilding, IP delay variability is typically quite low. For wide-spreadintranets, the delay variability could become too large. Then, more thanone time stamp sending units would be required.

[0055] Parts of the network part of the invention are preferablyimplemented by means of software run in a processor. Parts of thenetwork part of the invention can also be implemented as a hardwaresolution, for example by applying asic (Application Specific IntegratedCircuit) or separate logic.

[0056] The invention can also be described as a method illustrated inFIG. 4. The method starts in block 400. In block 402 is maintained thereference clock of the network part in the cellular radio network. Inblock 404 is generated a time stamp signal in the reference clock. Inblock 406 is transferred a time stamp signal over the asynchronous datatransmission connection from the network element to the base station. Inblock 408 is calculated on the basis of the transferred time stampsignal how much the time by the local clock at the base station deviatesfrom the time by the reference clock. In block 410 a speed correctionfactor is generated for the local clock at least from one calculateddeviation. In block 412 the running of the local clock is corrected byapplying the speed correction factor. In block 414 the frequenciesneeded at the base station are generated by using the local clockcorrected with the speed correction factor. The method is completed inblock 416.

[0057] Although the invention is described above with reference to anexample according to the accompanying drawings, it is apparent that theinvention is not restricted to it, but may vary in many ways within theinventive idea disclosed in the accompanying claims.

We claim:
 1. A network part in a cellular radio network comprising: abase station, the base station comprising a local clock; a networkelement connected to the base station via an asynchronous datatransmission connection, the network element comprising a referenceclock; the reference clock comprising means for generating a time stampsignal and means for sending the time stamp signal over the asynchronousdata transmission connection from the network element to the basestation; and the base station comprising means for receiving the timestamp signal sent over the asynchronous data transmission connection,means for calculating on the basis of the received time stamp signal howmuch the time by the local clock at the base station deviates from thetime by the reference clock, means for generating a speed correctionfactor for the local clock on the basis of at least one calculateddeviation, means for correcting the running of the local clock with thespeed correction factor, and a frequency synthesizer for generating thefrequencies needed at the base station by using the local clockcorrected with the speed correction factor.
 2. A network part accordingto claim 1, wherein the speed correction factor indicates when thesupply voltage of the local clock changes.
 3. A network part accordingto claim 2, wherein the speed correction factor compensates for changesthat gradually take place in the characteristics of the local clock. 4.A network part according to claim 3, wherein the speed correction factoris adjusted more than once a year.
 5. A network part according to claim1, wherein the base station further comprises means for calculating thedeviation in the transmission delay for the time stamp signal sent overthe asynchronous data transmission connection, for comparing thedeviation with a predetermined limit, and for concluding that theaccuracy of the local clock does not meet the required level if thedeviation exceeds the predetermined limit.
 6. A method for performingfrequency synchronization of a base station in a cellular radio network,the method comprising: maintaining a reference clock in a networkelement of the cellular radio network; generating a time stamp signal inthe reference clock; transferring the time stamp signal from the networkelement to the base station over an asynchronous data transmissionconnection; calculating on the basis of the transferred time stampsignal how much the time by the local clock at the base station deviatesfrom the time by the reference clock; generating a speed correctionfactor for the local clock on the basis of at least one calculateddeviation; correcting the running of the local clock with the speedcorrection factor; and generating the frequencies needed at the basestation by using the local clock corrected with the speed correctionfactor.
 7. A method according to claim 6, wherein the speed correctionfactor indicates when the supply voltage of the local clock at the basestation changes.
 8. A method according to claim 7, wherein the speedcorrection factor compensates for changes that gradually take place inthe characteristics of the local clock.
 9. A method according to claim8, wherein the correction is performed more than once a year.
 10. Amethod according to claim 6, wherein the deviation in the transmissiondelay for the time stamp signal sent over the asynchronous datatransmission connection is calculated, the deviation is compared with apredetermined limit, and if the deviation exceeds the predeterminedlimit it is concluded that the accuracy of the local clock at the basestation does not meet the required level.